Various motion assist devices configured to be worn by a user have been proposed for clinical purposes and improved life quality. Motion assist devices are often designed as walking assist devices for assisting the walking motion of the user, and power boosting devices for aiding a worker to work under heavy burdens or to perform other strenuous forms of work.
The motion assist device disclosed in JP2015-136623A is configured to assist the motion of the legs of the user, and uses a cable for transmitting the power. This motion assist device includes a pelvic support member configured to be worn on the pelvic part of the user, a pair of first frames each having one end connected to a corresponding one of the lateral ends of the pelvic support member via a pivot joint so as to be rotatable around a laterally extending axial line and configured to be secured to an upper leg of the user, a pair of second frames connected to the other ends of the respective first frames each via a pivot joint so as to be rotatable around a laterally extending axial line and configured to be secured to the respective lower legs of the user, and actuators mounted on the pelvic support member. The power of the actuators are transmitted to the various pivot joints each via a pulley and cable power transmission mechanism.
According to this prior art, a same number of actuators as the number of the joints to be assisted are required. Furthermore, when the user performs a motion such as sitting and stooping which is not assisted by the device, the user encounters a considerable resistance caused by the actuators which are turned off. As the actuators typically consist of permanent magnet motors, even when the actuators are turned off, a considerable torque is required to overcome the resistance of the actuators. In particular, when a gear reduction mechanism is provided in each actuator, it may be impossible for the user to rotate or otherwise move the output end of the actuator.